The present invention relates to a display device, and more particularly, to a liquid crystal display device or an organic EL display device capable of detecting the degree of a crack in a cutting-plane line of a glass substrate, and an impact of the crack.
The liquid crystal display device includes a TFT substrate having pixel electrodes and thin film transistors (TFT) formed in a matrix, and a counter substrate having a black matrix or an overcoat film formed at a location corresponding to the pixel electrode of the TFT substrate. The liquid crystal is interposed between the TFT substrate and the counter substrate. Images are formed by controlling light transmittance of the liquid crystal molecules for each pixel.
Especially, the liquid crystal display devices of medium and small sizes are strongly demanded to expand the display region while keeping the designed outer shape. In this case, the width defined by edges of the display region and a liquid crystal display panel, that is, a frame region is reduced. It is necessary to provide wirings such as scanning lines, video signal lines, and common wirings at an outer side of the display region. In order to ensure the aforementioned wiring region, the wiring has to be provided to the lower area of the sealing material for bonding the TFT substrate and the counter substrate.
Manufacturing the liquid crystal display devices of medium and small sizes one by one is disadvantageous in terms of cost. For this reason, the manufacturing process includes the step of forming a plurality of liquid crystal display devices on a large board, and separating the completed liquid crystal display devices respectively. In other words, a plurality of TFT substrates are formed on a large mother TFT board, and a plurality of counter substrates are formed on a large mother counter board. The mother TFT board and the mother counter board are bonded with each other to constitute a mother board. The resultant mother board is subjected to scribing, and is exposed to the impact for fracture so as to separate the board into the respective liquid crystal display panels.
When the glass is subjected to fracture, a crack may occur in an area around the liquid crystal display panel. If the panel has the small frame and wiring such as the leader is formed at the area around the edge of the liquid crystal display panel, the crack may disconnect the peripheral leader. Accordingly, the wiring for detecting the crack is provided on the inner circumference of the liquid crystal display panel.
For inspecting the liquid crystal display panel, various types of inspections have to be carried out, for example, pixel lighting inspection, disconnection inspection of the wiring and the like in addition to the crack detection. A large number of inspection terminals are required in accordance with all the inspections as described above, resulting in insufficient space for the terminal portion of the liquid crystal display panel. Japanese Unexamined Patent Application Publication Nos. 2007-171993, 2008-9246, and 2009-92965 disclose the structure configured to use the switching transistor so as to reduce the number of the inspection terminals.
FIG. 7 is a plan view representing an example of a liquid crystal display device to which the present invention is applied. Referring to FIG. 7, a liquid crystal (not shown) is interposed between a TFT substrate 100 and a counter substrate 200. The TFT substrate 100 is made larger than the counter substrate 200, and an extended part of the TFT substrate is used as a terminal portion 120 on which an IC driver 40 is mounted and various terminals are formed. The terminal portion 120 is connected to a flexible wiring substrate 110.
Referring to FIG. 7, a polarizing plate 210 is bonded to the counter substrate 200. A polarizing plate 210 (not shown) is also bonded to the back surface of the TFT substrate 100. A display region 300 is formed slightly inside the polarizing plate 210. As FIG. 7 shows, a crack detection line CRW which runs along the edge side of the TFT substrate 100 is indicated by a bold line for easy identification. GND in the flexible wiring substrate 110 is a terminal for ground.
Cracks exist in all the glass substrates. The problem may occur when such crack develops to disconnect the wiring, or to deteriorate reliability of sealing between the TFT substrate 100 and the counter substrate 200. Therefore, the crack detection line CRW serves to detect whether or not the crack is larger than a predetermined size. Hereinafter, unless otherwise specified, the determination with respect to existence of the crack refers to the existence of the crack equal to or larger than a predetermined size.
Referring to FIG. 7, a current is supplied via the flexible wiring substrate 110 to detect the existence of the crack. If the crack detection line CRW is disconnected by the crack, the current is no longer applied. Therefore, it is possible to detect the existence of the crack based on the current flow.
FIG. 8 schematically shows a generally employed crack detection circuit of the liquid crystal display device as shown in FIG. 7. FIG. 8 shows a magnified area of the terminal portion on which various terminals are formed for easy identification. Actually, the display region 300 shown in FIG. 8 has TFTs for the respective pixels. However, they are omitted for avoiding complexity. FIG. 8 omits the scanning lines. Actually, however, the scanning line is connected to the IC driver 40, and supplies the signal for turning the TFT of the pixel ON upon crack detection. If the scanning signal circuit is built in the liquid crystal display panel without using the IC driver, the scanning line is connected to the built-in scanning signal circuit.
The display region 300 is formed on the counter substrate 200 overlapped with the TFT substrate 100. The display region 300 has longitudinally extending video signal lines 50 which are transversely arranged. The video signal lines 50 are drawn to the terminal portion 120, and connected to a red pixel terminal R, a green pixel terminal G and a blue pixel terminal B, respectively via a lighting inspection switching transistor 10.
A gate line for turning the lighting inspection switching transistor 10 on/off is connected to a first test gate terminal TG1 and a second test gate terminal TG2. The lighting inspection switching transistor 10 is turned on upon lighting inspection, and off upon normal image formation. As FIG. 8 shows, the lighting inspection switching transistor 10 disposed at the same location as the IC driver 40 is actually formed on the TFT substrate 100 not on the IC driver 40.
Referring to FIG. 8, the crack detection line CRW runs along the side of the TFT substrate 100 apart from its edge by a distance of d. The distance d may be set to 200 μm, for example. If the crack becomes deeper than 200 μm, the crack detection line CRW is disconnected to block the current flow. As a result, the subject liquid crystal display panel is regarded as a defective product.
The detection method represented by FIG. 8 is required to carry out the inspection by applying the current to the crack detection line CRW. It is necessary to connect the flexible wiring substrate 110 as shown in FIG. 7 for applying the current. That is, the generally employed method is not capable of detecting the existence of the crack until the flexible wiring substrate 110 is connected.